In "Chickenpox", the chickenpox virus spreads throughout South Park and Shelly Marsh is infected. Sharon Marsh takes her to see Dr. Doctor for treatment; however, he recommends children should come into contact with chickenpox at a young age, as the virus is more dangerous if a person is infected at an older age.
Upon hearing this advice, the boys' parents scheme to get their children infected, by having them play with Kenny McCormick, who also is sick with chickenpox. Kyle Broflovski is the only child not affected by the virus, and appears to be immune to it. His mother, Sheila Broflovski, sends him to Kenny's house, in hopes of him becoming infected.